Terminology

Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than, say, ten metres across being called a meteoroid. The term "asteroid" is somewhat ill-defined. It has never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union until 2006, when the term "small Solar System body" was introduced. Other languages prefer "planetoid" (Greek for "planet-like"). The word "planetesimal" has a similar meaning, but often refers specifically to small bodies that existed at the time the Solar System was forming. The term "planetule" was coined by the geologist Conybeare to describe minor planets, but is not in common use.

When discovered, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets eventually are depleted of their volatile ices and then appear as point-like objects, i.e. asteroids. A further distinction is that comets typically have more eccentric orbits than asteroids (though some objects classified as asteroids also have notably eccentric orbits).

For almost two centuries, from the discovery of the first asteroid, 1 Ceres, in 1801 until the discovery of the first centaur, 2060 Chiron, in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo ventured far beyond Jupiter for part of their orbit.

When astronomers started finding small bodies that permanently resided further out than Jupiter, now called centaurs, they were counted alongside the asteroids, though there was debate over whether they should be classified as asteroids or as a new type of object. Then, when the first trans-Neptunian object, 1992 QB1, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper Belt object (KBO), trans-Neptunian object (TNO), scattered-disc object (SDO), and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity.

The innermost of these are the Kuiper Belt Objects (KBOs), called "objects" partly to avoid the need to classify them as asteroids or comets. KBOs are believed to be predominantly comet-like in composition, though some may be more akin to asteroids. Furthermore, they do not necessarily have the highly eccentric orbits usually associated with comets, and there are significant numbers very much larger than traditional comet nuclei. The much more distant Oort cloud is also hypothesized to be a reservoir of dormant comets. Other recent observations, such as the analysis of the cometary dust collected by the Stardust probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line.